This invention relates to a safety device for use in a nuclear reactor. More particularly, this invention relates to a safety device for use with a control rod in a nuclear reactor such that in the event that the temperature of the reactor core rises above a normal operating level the device will axially reposition the control rod with respect to the reactor core.
A nuclear reactor typically includes a reactor vessel containing a core, said core comprising vertical elongated fuel assemblies and interspersed control rods. In a liquid metal fast breeder reactor (LMFBR) the fuel assemblies are usually supported from beneath the core, while the control rods ae suspended from control rod drive means disposed above the core. The fuel assemblies are disposed to provide a critical mass of nuclear fuel within the core so that a nuclear chain reaction is sustained and nuclear energy is produced. Coolant flows upward from the bottom of the reactor vessel, past the fuel assemblies and control rods, and out of the vessel, carrying energy produced by the chain reaction as heat to a heat exchange system.
The nuclear chain reaction is moderated by means of the control rods. The control rods may contain either a nuclear fuel, a neutron poison, or some combination of fuel and poison, depending on the design of the particular reactor. Regardless of composition, control rods are typically designed so that upward motion of the rods accelerates the chain reaction, and downward motion of the rods slows the chain reaction. The control rods may be individually axially repositioned as required to sustain the chain reaction by the control rod drive means. Under normal operating conditions the control rod drive means reposition the control rods at a very slow rate, on the order of an inch per minute. This slow rate maintains the chain reaction equilibrium and minimizes the thermal stresses in the reactor.
For maximum safety, nuclear reactors are designed to anticipate and respond appropriately to breakdowns due to either equipment malfunctions or natural disasters such as earthquakes. Under such emergency conditions, the control rod drive means are designed to rapidly force the control rods down into the reactor core at a rate of several feet per second. This rapid movement, known as a reactor "scram," immediately stops the nuclear chain reaction. Thus, if an upward thermal excursion should raise the coolant temperature above its normal safe operating level, the control rod drive mechanisms are activated automatically to scram the reactor. However, it would be desirable for the reactor to be safe even if the primary automatic safety system were to fail to activate, as may occur in a hypothetical "loss-of-flow-without-scram" incident. For that type of incident, it would be desirable to have a supplemental system which would drive the control rods downward into the core to stop or slow the chain reaction. Preferably, such a system would operate passively without external activation. Ideally, such a system would operate on the basis of physical principles and would require no moving mechanisms.